The present invention relates to a device for the reception and/or the transmission of electromagnetic signals which can be used in the field of wireless transmissions, in particular in the case of transmissions in an enclosed or semi-enclosed environment such as domestic environments, gymnasia, television studios or auditoria, etc.
In the known systems for high-throughput wireless transmissions, the signals sent by the transmitter reach the receiver along a plurality of distinct routes. When they are combined at receiver level, the phase differences between the various rays which have travelled routes of different length give rise to an interference figure liable to cause fadeouts or a considerable degradation of the signal. Moreover, the location of the fadeouts changes over time as a function of the modifications of the surroundings, such as the presence of new objects or the passage of people. These fadeouts due to multipaths may engender considerable degradations both as regards the quality of the signal received and as regards the performance of the system.
To remedy the problem of fadeouts relating to multipaths, use is currently made of directional antennas which, through the spatial selectivity of their radiation patterns, make it possible to reduce the number of rays picked up by the receiver, thus attenuating the effect of the multipaths. In this case, several directional antennas associated with signal processing circuits are required to ensure spatial coverage of 360xc2x0. French Patent Application No. 98 13855 filed in the name of the applicant also proposes a compact multibeam antenna making it possible to increase the spectral efficiency of the array. However, for a number of items of domestic or portable equipment, these solutions remain bulky and expensive.
To combat fadeouts, the technique most often used is a technique using space diversity. As represented in FIG. 1, this technique consists among other things in using a pair of antennas with wide spatial coverage such as two antennas of the patch type (1, 2) which are associated with a switch 3. The two antennas are spaced apart by a length which must be greater than or equal to xcexO/2 where xcexO is the wavelength corresponding to the operating frequency of the antenna. With this type of device, it can be shown that the probability of the two antennas being simultaneously in a fadeout is very small. The proof results from the description given in xe2x80x9cWireless Digital Communicationxe2x80x9d, Dr Kamilo Feherxe2x80x94chapter 7xe2x80x94Diversity Techniques for Mobile-Wireless Radio Systems, in particular from FIG. 7.8, page 344. It can also be proven through a pure probability calculation with the assumption that the levels received by each patch are completely independent. It can be stated, in this case, that if p (1% for example) is the probability that the signal received by an antenna has a level lower than a detectability threshold, the probability that this level is below the threshold for the two antennas is p2 (hence 0.01%). If the two signals are not perfectly uncorrelated, then pdiv is such that 0.01% less than pdiv less than 1%, where pdiv is the probability that the level received is lower than the detectability threshold in the case of diversity. Moreover, by virtue of the switch 3, it is possible to select the branch linked to the antenna exhibiting the highest level by examining the signal received by way of a monitoring circuit (not represented). The antenna switch 3 is connected to a switch 4 making it possible to operate the two patch antennas 1 or 2 in transmission mode when they are linked to the Txc3x975 circuit or in reception mode when they are linked to the Rxc3x976 circuit.
The aim of the present invention is to propose an alternative solution to a conventional solution of the type described above, which applies to antennas of the slot-fed type and which makes it possible to obtain radiation diversity.
The aim of the present invention is also to propose a solution making it possible to preserve quasi-omnidirectional azimuthal coverage.
In consequence, the subject of the present invention is a device for the reception and/or the transmission of electromagnetic signals comprising at least two means of reception and/or of transmission of electromagnetic signals of the slot-fed antenna type and means of connection for connecting at least one of the said means of reception and/or of transmission to means of utilization of the multibeam signals,
characterized in that the means of connection consist of two feed lines connected by a connection element to the utilization means, the two lines being coupled electromagnetically with the slots of the slot-fed antennas, each line terminating in a switching element arranged in such a way as to simulate, as a function of a monitoring signal, an open circuit or a short circuit at the extremity of one of the lines and a short circuit or an open circuit at the extremity of the other line so as to obtain different radiation patterns.
According to a preferred embodiment, the slot-fed antennas are antennas of the Vivaldi type regularly spaced around a central point. Moreover, the feed lines consist of microstrip lines or of coplanar lines.
In accordance with the present invention, the feed lines cross the slot-fed antennas in an open-circuit zone in respect of the slots.
According to another embodiment, the feed lines cross the slots of the slot-fed antennas in two distinct open-circuit planes of the slot. Moreover, the length of the first feed line between two slots of the slot-fed antennas is equal to kxcexl and the length of the second feed line between two slots of the slot-fed antennas is equal to (k+0.5)xcexl where xcexl is the wavelength guided in the line and k is a positive integer.
According to a preferred embodiment, the switching element consists of a diode. The connection element consists of a T element dimensioned to send the energy selectively to one or the other feed line. Hence, the length of the feed line between the slot of the slot-fed antenna and the T is equal to I=xcexl/2 with n integer, and xcexl the wavelength guided in the line.